Study of the response behavior of a CdS–SnO2 thick film for high selectivity towards propanol gas

Gas monitoring devices are in demand for a rapidly growing range of applications. Metal oxide-based gas sensors have been extensively used for the detection of toxic pollutant gases, combustible gases, and hydrocarbon vapors. The sensitivity for a low concentration and observed response and the recovery times of the reported gas sensors are not satisfactory, and it needs further detailed studies. In the present work, undoped SnO2 and cadmium sulfide (CdS)-doped SnO2 thick films were fabricated using the screen-printing method to study their sensing behavior towards tested organic vapors such as acetone, propanol, and ethanol. The sensing properties of fabricated sensors were investigated for the test gases, i.e. acetone, propanol, and ethanol, at an elevated temperature of 473 K. It was observed that the 2 wt% CdS-doped SnO2 sensor showed a maximum response (78%) and was highly selective (44.6%) to propanol over acetone and ethanol. The results showed that the diminution of the SnO2 crystallite size with the CdS content leads to an improvement in the response of the SnO2 sensor for the tested gases. The microstructural properties are also correlated to the sensing behavior. The measurement showed that the CdS–SnO2 thick film sensor is highly sensitive. At the same time, it is more selective to propanol than the other test gases, ethanol and acetone.


Introduction
2][3][4] Semiconducting oxides are classied as n-type and p-type.Currently, p-type semiconducting materials are required for excellent application of the sensor.In particular, SnO 2 is usually regarded as an oxygen-decient n-type semiconducting material. 5][8][9][10][11] In the sensor technology, the 3S rule is applied, i.e., S = sensitivity, S = selectivity, S = stability.3][14][15] SnO 2 is used in sensors of combustible gases including carbon monoxide detectors.In these sensors, the sensor area is heated to a constant temperature (a few hundred °C), and in the presence of a combustible gas, the electrical resistivity drops. 16Doping with various compounds has been investigated (e.g., with CuO). 17Doping with cobalt and manganese gives a material that can be used in high voltage, for example; in addition, tin(IV) oxide can be doped with the oxides of iron or manganese. 18J.K. Srivastava et al. 19 reported on the microstructural proprieties of the PbO-doped SnO 2 sensor for the detection of methanol, propanol, and acetone.They observed that at 3wt% PbO-SnO 2 gives the maximum response for propanol at 350 °C.They also described the optimization of the ring temperature of the doped SnO 2 sensor for methanol and acetone detection. 12F. H. Saboor et al. 13 presented the NO 2 gas sensing for Pd-loaded SnO 2 thick lm gas sensors under UV light irradiation at room temperature.Xi-Tao Yin et al. 15 studied the selectivity and sensitivity of a Pd-loaded Fe-doped SnO 2 sensor for CO detection in the presence of H 2 , and showed that 10 mol% Fe, 0.2 mol% Pd gives the maximum sensitivity and selectivity to CO against H 2 .M. Choudry et al. 20 reported the effect of temperature on a Pd-doped SnO 2 thick lm gas sensor and observed that the ring temperature at 800 °C gives a more selective sensor for the detection of LPG, H 2 , and CH 4 , and the reported selectivity of LPG with a maximum sensitivity of up to 87%.In a previous paper, Yadava et al. 21reported on the sensing properties of the CdS-doped SnO 2 thick lm gas sensor for the detection of methanol, and showed that the 2wt% CdS-doped SnO 2 sensor is a suitable detector for methanol.In the present study, we report the microstructural properties and sensing response of the CdS-doped SnO 2 thick lm.The reduction in crystallite size with the addition of the lower CdS content leads to an improvement in the response of the SnO 2 sensor for the target gases, propanol, methanol, and acetone.The microstructural properties are correlated with the sensing RSC Advances PAPER behavior of the undoped and doped SnO 2 sensors.It is observed that the 2 wt% CdS doped SnO 2 sensor showed a maximum response (78%) and was highly selective (44.6%) to propanol over acetone and ethanol.

Synthesis and fabrication of the sensor
In the laboratory, we take tin oxide (SnO 2 ) powder and glass binder (10 wt% of SnO 2 ), and mix them properly using a ball mill (Zirconia Ball Mill, Retsch) for 3-4 hours. 21Fine grains are then mixed with an organic binder (diethyl glycol mono butyl) and organic solvent (a-terpinol) in a ball mill for 1-2 h, which results in an undoped SnO 2 paste.For doped pastes of SnO 2 , weighed SnO 2 powder with glass binder (10 wt% of SnO 2 ) and CdS powder (1 wt%, 2 wt%), all of these components are mixed in a ball mill, and the same organic binder and solvent are used to obtain the sensing paste.The prepared paste is screenprinted on an alumina substrate (25 mm × 25 mm) with a nger electrode pattern on the front side and a resistor heater pattern on the backside, as shown in Fig. 1.

Characterization and sensing setup
XRD analysis was performed on the structural and lattice parameters of the S 1 , S 2 , and S 3 lms.The D8-Advanced apparatus, connected with a source of Cu Ka 1 radiation with a wavelength of 0.15406 nm, produced the XRD pattern.For the measurement of the response of the fabricated sensors, S 1 , S 2, and S 3 , a test chamber has been made in which provisions are made for the electrical connection and inlet/outlet for test gases.The connectivity for the voltage supply and resistance measurement is available through insulated gaskets on the base of the chamber.The resistance of the fabricated sensors in the air and gas environments is measured with the help of a Dual DC power supply (LD-3202) and Digital Multimeter (Aplab 107N).The response of the fabricated sensors is measured in ambient air with varying concentrations of acetone, propanol, and ethanol gases (0-5000 ppm) separately at 473 K.

XRD
The experimental XRD pattern of samples S 1 , S 2 , and S 3 is shown in Fig. 2. The XRD pattern of the fabricated samples has been performed in a wide range of diffraction angles from 0°to 90°.The comparative study of the XRD pattern for the sensors S 1 , S 2 , and S 3 was performed in diffraction angles from 20°to 55°only.At an angle of 2q, the peaks of the diffraction pattern correspond to the planes (110), ( 101), ( 200), ( 210), ( 002), (122), and (301), respectively.The polycrystalline structure of the deposited lm is conrmed by these peaks and the correlating planes.If we increase the concentration of CdS in the sensors S 2 and S 3 , modication in the XRD pattern is obtained.As we can see from the magnied gure at the plane (200) in Fig. 2(b), the peaks shi toward a higher value of 2q.As the concentration of CdS increases in the sample, the peak intensities decrease and the peaks are broadening.The crystalline size can be calculated by the well-known formula given by the Debye Scherer relationship. 22,23¼ lk b cos q (1) In this case, k = 0.94, where l is the X-ray source's wavelength, b is the full width at half maximum (FWHM), and q is the diffraction angle.The crystallite sizes of samples S 1 , S 2 , and S 3 are found at 25.1 nm, 16.1 nm, and 13.9 nm, respectively.This shows that the crystallite size of the SnO 2 lms decreases with increasing CdS concentration.
To calculate microstrain, we can use the formula given below: The calculated microstrain increases with the increase of the CdS concentration.The crystallite size, rms roughness, and microstrain versus the CdS concentration are shown in Fig. 3(a  and b), and the calculated value of the microstrain is 0.47 nm, 0.57 nm, and 0.61 nm for S 1 , S 2 , and S 3 .Also, we observed that the crystallite size and roughness decrease with an increase in the CdS contents (Fig. 3(a)).

Gas sensing response
The sensitivity is dened as the slopes of response vs. the gas concentration in ppm.Sensitivity is calculated by the following formula: 25,26 Sð%Þ ¼ where R a is the resistance in clean air and R g is the resistance in the presence of vapors.The response of the fabricated sensors,

Selectivity
Selectivity is one of the most important parameters of the sensing properties of the gas sensor.The selectivity of the sensor was calculated using the following formula: 27,28 where S i is the response of gas for which the relative is to be estimated.The responses were found to be ∼45.63%,∼33.91%, and ∼20.46% for propanol, ethanol, and acetone, respectively, which is displayed in the histogram (Fig. 5).It is evident from for test gases at 473 K.At time 10 min, 5000 ppm acetone was injected in the test chamber to get the maximum response, and then the chamber was opened to recover the initial state of the sensor.The time corresponds to the time required for the gas response to reach 90% of the nal equilibrium value aer a test gas is injected, and the recovery time is the time recorded for the gas response to decrease by 90% its maximum value when the gas sensor is exposed in ambient air. 30,31Fig. 6(b) shows that in the S 3 samples, the fast response and recovery times decrease from 31 s to 19 s and 89 s to 67 s for propanol (5000 ppm at 200 °C), respectively.The transient response and recovery curve to the acetone and ethanol sensors S 1 and S 3 is plotted in Fig. 6(a and c), and its value is listed in Table 1.

Sensing mechanism
The sensing mechanism of the fabricated sensors (S 1 , S 2 , S 3 ) can be realized as the resistance of materials varies with oxygen molecules absorbed at the surface and with the amount of   reducing gas species.When metal oxide comes in contact with air, the oxygen molecules from the ambient environment are absorbed onto the surface of the metal oxide at the grain boundaries, which trap electrons and build a barrier around each grain. 32The adsorbed oxygen molecules were subsequently converted to ions aer capturing an electron from the conduction band.The adsorbed oxygen exists on the surface in the form of O − , O 2

−
, and O 2− species, depending on the temperature. 33The chemical absorption process can be explained by the following reaction (eqn ( 5)-( 8)): When sensor S 3 contacts with the reducing gas, the reducing gas propanol (C 3 H 7 OH) reacts with the oxygen species to produce carbon dioxide and water, and the resulting electron will return to the conduction band of the semiconductor.The possible surface reactions take place (Fig. 7): Among the adsorbed oxygen species (O 2 −, O 2− , and O − ) on the surface, the majorly atomic species (O − single ionized oxygen) is more reactive, particularly at higher temperatures (150 °C < T < 400 °C).When the CdS-SnO 2 sensor is exposed to the target gases, propanol, ethanol, and acetone, at an elevated temperature of 200 °C, the gas will be adsorbed on the surface of the sensor.These gases will react with the chemically adsorbed oxygen anions, particularly O − .The electrons are released back to the material of cadmium sulde, resulting in a decrease in the depletion layer thickness on the surface of the sensor, and also a decrease in the surface band bending.Therefore, the change in the electrical resistance of the sensor is observed with exposure to target gases.Among the target gases, the S 3 sensor showed a larger response and at the same time high selectivity, which may be due to its different molecular structures.Also, it is obvious from eqn (9) that the carrier's concentration increased, which caused the sensor resistance to be reduced.Therefore, this process will increase the carrier concentration on the surface of tin oxide, resulting in a decrease in resistance. 34For a denite understanding regarding the high selectivity, future work is to be done using an array of sensors and exposure to a mixture of target gases.
We compare our ndings to other related work, and a comparison table of the sensing response, operating temperature, and response time for alcoholic and acetone gas is listed in Table 2.

Conclusion
Three types of sensors, namely S 1 (undoped SnO 2 ), S 2 (1 wt% CdS-SnO 2 ), and S 3 (2 wt% CdS-SnO 2 ), are fabricated.It is found from the XRD pattern that the crystallite size and roughness decrease with dopant CdS (0-2%) and it lies within the anemometric range.The diminution of the SnO 2 crystallite size leads to an improvement in the sensitivity of the fabricated sensors for test gases.Microstructural properties are correlated with the sensitivity of the materials and dopants.The sensing behavior of the sensors S 1 , S 2 , and S 3 operated at 200 °C for the different target gases (propanol, acetone, and ethanol) under ambient air is reported.The resistance of the sensor decreases with the increasing concentration of gases (0-5000 ppm).It is observed that the 2 wt% CdS-doped SnO 2 sensor showed the greatest response (78%), and is highly selective (44.6%) to propanol over acetone and ethanol with a fast response and recovery time.Thus, we conclude that the CdS doped-SnO 2 thick lm sensor is highly sensitive.Furthermore, it is more selective to propanol gas than the other target gases.

Fig. 1
Fig. 1 Finger electrode and heater pattern of the sensor.

S 1 ,
S 2 , and S 3 , are measured with varying concentrations (0-5000 ppm) of the target gases, acetone, ethanol, and propanol, at a xed operating temperature of 200 °C.The response with the concentration of acetone for sensors S 1 , S 2 , and S 3 is shown in Fig.4(a).The response increases with increasing concentration of the acetone vapors.The sensitivity is 36%, 31%, and 25% for the S 3 , S 2 , and S 1 sensors, respectively.It is evident from Fig.4(b) that the maximum response for propanol was achieved with the S 3 sensor (78%); it is (1.14) times that of S 2 and (2.79) times that of S 1 .The response curve of ethanol is shown in Fig.4(c).The sensitivity of the fabricated sensor has been reported

Fig. 4
Fig. 3 (a) Variation of the CdS content vs. crystallite size, rms roughness.(b) Variation of the CdS content vs. microstrain.

Fig. 4
Fig. 4 Sensing response with standard deviation error bars for (a) acetone, (b) propanol, and (c) ethanol gas for sensors S 1 , S 2 , and S 3 .(d) Comparative response with the standard deviation error bars of sensor S 3 .

Fig. 5
Fig. 5 Selectivity of ethanol, acetone, and propanol gas with standard deviation error bars for sensor S 3 .
in Table1.Fig.4(d)represents the comparison curve of the S 3 sensor for acetone, ethanol, and propanol.It is observed that sensor S 3 is more sensitive to propanol gas than acetone and ethanol.The sensitivity of propanol for sensor S 3 is 1.34 times that for ethanol and 2.16 times that for acetone.Therefore, propanol has a maximum response compared to the other test gasses.The standard deviation of the response of sensors S 1 , S 2 , and S 3 for acetone is 5.1, 9.7, and 11.2, respectively.The standard deviation of the response of sensors S 1 , S 2 , and S 3 for propanol gas is 8.33, 16.97, and 18.87, respectively.The standard deviation of the response of sensors S 1 , S 2 , and S 3 for ethanol gas is 14.85, 14.01, and 14.46, respectively.

Table 1
Sensing response and selectivity and transient time of the S 3 sensor for propanol, ethanol, and acetone

Table 2
A comparison table of the sensing response, operating temperature, and response time for alcoholic and acetone gas